Updating this thread in case someone comes across this issue on their own file... given two AAM users have seen the same thing happen, there is likely to be more.
Revenue have responded to say that 'After reviewing your taxes, it appears that two coded underpayments for 2021 were cancelled on 31/08/2022. Consequently these payments were not materialised. The payments amounted to €xxx.xx each, or €yyy.yy in total which is the balance remaining for 2021.'
However, when I look at my statement of liability for 2023 and 2022, the 2023 underpayment (i.e. reduction in credits) is exactly this amount, and my tax credit cert for 2023 (issued December 2022) has the same reduction in credits, and this occurred after this supposed cancellation. My 2022 tax credit reduction is actually higher than the 2023 amount, but I assume this includes a liability reclaim by Revenue for another period. It's not possible to see this.
So I'm going back to Revenue about this again to see if they can say for what periods these payments didn't materialise.